This invention relates to an antenna apparatus and, in particular, to an antenna apparatus including a flat-plate radiation element.
Generally, an antenna apparatus of the type comprises a dielectric substrate, a flat-plate radiation element disposed on the dielectric substrate, and a feeding lead connected to the radiation element and extracted outward through the dielectric substrate. Such antenna apparatus is disclosed, for example, in Japanese Patent Application Publication (JP-A) No. 2002-198725. Referring to FIG. 1, this antenna apparatus comprises a flat-plate radiation element 11 of a generally square shape disposed on an upper surface of a dielectric substrate 12 of a generally rectangular shape, a one-point feeding member 13 located at an offset position of the radiation element 11, and a grounding conductor member 14 having a generally rectangular shape and attached throughout an entire region of a lower surface of the dielectric substrate 12. Although not shown in the figure, a ground electrode is adhered to the grounding conductor member 14. A combination of the radiation element 11 and the dielectric substrate 12 forms a patch antenna. With the above-mentioned structure, it is possible to set the best axial ratio for a signal frequency of a circular polarization signal to be transmitted and received.
Another existing antenna apparatus is disclosed, for example, in Japanese Patent Application Publication (JP-A) No. 2002-237714. As shown in FIG. 2, this patch antenna comprises a flat-plate radiation element 21 disposed on an insulation substrate 22, a feeding member 23 located at an offset position, a ground conductor 24 disposed throughout an entire region of an upper surface of the insulation substrate 22, and a dielectric member 25 placed on the ground conductor 24 at a position where an electric field strength is weak. The radiation element 21 is spaced from the ground conductor 24 by the presence of the dielectric member 25 interposed therebetween. With the above-mentioned structure, it is possible to obtain a patch antenna with higher gain.
Still another existing antenna apparatus using a circular polarization antenna is disclosed, for example, in Japanese Patent Application Publication (JP-A) No. 2001-339234. For example, in a circular polarization antenna for use in GPS (Global Positioning System), a flat-plate radiation element has a six-sided shape formed by trimming a square shape by cutting off two corners opposite to each other along a diagonal line.
As shown in FIGS. 3 and 4A and 4B, the antenna apparatus comprises a radiation element 31 disposed on an upper surface of a dielectric substrate 32, a feeding lead 33 located at an offset position of the radiation element 31, and a ground conductor 34 attached to a lower surface of the dielectric substrate 32. The feeding lead 33 passes through the dielectric substrate 32 and the ground conductor 34 to protrude outward.
The patch antenna or the antenna apparatus described in conjunction with FIGS. 1 to 4B is broadly adopted in an on-vehicle or a pocket navigation system of the GPS system using an electrical wave received from a satellite, i.e., a satellite wave.
In recent years, a digital radio receiver, which receives the satellite wave or the ground wave so as to listen to digital radio broadcasting, has been developed and is put into practical use in the United States of America. The digital radio receiver is mounted on a mobile station, such as an automobile, and can receive an electric wave having a frequency of about 2.338 gigahertz (GHz) to listen to the digital radio broadcasting. That is, the digital radio receiver is a radio receiver adapted to listen to mobile broadcasting. In addition, the ground wave is an electric wave obtained by slightly shifting the frequency of the satellite wave after it is received by an earth station.
In order to receive the electric wave having the frequency of about 2.338 GHz, it is necessary to dispose an antenna outside the automobile. A variety of types of antennas having various structures have been proposed. Generally, cylindrical antennas are frequently used rather than planer or flat antennas. This is because a wider directivity is achieved by forming the antenna into a cylindrical shape.
As well known in the art, an electromagnetic wave radiated in a free space is a transverse wave having electric and magnetic fields which oscillate at right angles with respect to each other within a plane perpendicular to a traveling direction of the wave. Each of the electric field and the magnetic field is variable in strength within the above-mentioned plane. The electromagnetic wave having the above-mentioned feature is called a polarized wave or polarization. Thus, the polarized wave is an electromagnetic radiation in which the direction of the electric field vector is not random. The satellite wave is a circular polarization while the ground wave is a linear polarization.
As described above, the on-vehicle or the pocket antenna apparatus, which can be mounted at any position and does not protrude from a body of the automobile or a housing of a mobile equipment, is wide spread for use in the GPS system. Moreover, it is desired that such antenna apparatus is effectively applied also to the digital radio broadcasting.
However, each of the existing patch antennas or the existing antenna apparatuses described above has a radiation characteristic intended to the GPS system accommodating a limited number of satellites. In other words, each of the antennas described above does not have such a wide directivity achieved by the cylindrical antenna and adaptable to the digital radio broadcasting. As a result, the above-mentioned antenna having the flat-plate radiation element is disadvantageous in that it is unsuitable for the digital radio broadcasting.